Baeyer Strain Theory: Stability of Cycloalkanes | Chemistry for JEE & NEET

Baeyer Strain Theory: Stability of Cycloalkanes

In organic chemistry, the stability of cycloalkanes has always been a fascinating topic. Why is cyclopropane highly reactive while cyclopentane is relatively stable? The answer lies in the Baeyer Strain Theory, proposed by Adolf von Baeyer in 1885. This theory is a cornerstone concept for students preparing for JEE and NEET.

1. The Core Postulates

Adolf von Baeyer based his theory on the assumption that carbon atoms in cycloalkanes prefer a tetrahedral geometry with a normal bond angle of 109.5° (109° 28'). Any deviation from this ideal angle creates an internal strain known as Angle Strain.

• Carbon atoms form a planar ring (Note: This was later proven incorrect for larger rings, but it is a key postulate of this theory).
• If the bond angle deviates from 109.5°, the molecule becomes unstable.
• Higher the angle strain, lower the stability, and higher the reactivity (Heat of Combustion).

2. Calculating Angle Strain

The angle strain (α) can be calculated using the following formula, where $n$ is the number of carbon atoms in the ring:

Angle Strain (α) = ½ [ 109°28' - Internal Bond Angle ]

The internal bond angle of a regular polygon is calculated as: $ \text{Angle} = \frac{180(n-2)}{n} $

Angle Strain Data for Cycloalkanes

Compound	Hybridization	Internal Angle	Angle Strain	Stability Status
Cyclopropane	sp3	60°	+24° 44'	Highly Unstable
Cyclobutane	sp3	90°	+9° 44'	Unstable
Cyclopentane	sp3	108°	+0° 44'	Stable
Cyclohexane	sp3	120°	-5° 16'	Stable (By Sachse-Mohr)

3. Stability Order

According to Baeyer, Cyclopentane should be the most stable ring because its bond angle (108°) is closest to the ideal tetrahedral angle (109.5°). The predicted stability order by Baeyer was:

Cyclopentane > Cyclohexane > Cyclobutane > Cyclopropane

4. Limitations of Baeyer Strain Theory

While Baeyer explained smaller rings well, his theory failed for larger rings (n ≥ 6).

1. Cyclohexane is actually more stable than Cyclopentane. Experimental heat of combustion data proves cyclohexane is strain-free.
2. Baeyer assumed rings are planar (flat). In reality, rings with 6 or more carbons adopt puckered (non-planar) conformations (like Chair and Boat forms) to relieve strain.
3. This led to the Sachse-Mohr Theory of strainless rings.

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Test Your Knowledge: Baeyer Strain Theory Quiz

Select the correct answer for the following questions. Click submit to see your score and detailed explanations.

1. According to Baeyer Strain Theory, which cycloalkane is predicted to be the most stable?

a) Cyclopropane b) Cyclobutane c) Cyclopentane d) Cyclohexane

Answer: c) Cyclopentane.
Baeyer predicted Cyclopentane (angle 108°) is most stable as it is closest to 109.5°. (Note: In reality, Cyclohexane is more stable, but the question asks "According to Baeyer").

2. What is the ideal tetrahedral bond angle for sp³ hybridized carbon?

a) 120° b) 109° 28' c) 90° d) 180°

Answer: b) 109° 28'.
This is the standard angle for a regular tetrahedron and sp³ hybridization.

3. What is the approximate angle strain in Cyclopropane?

a) 24° 44' b) 9° 44' c) 0° 44' d) -5° 16'

Answer: a) 24° 44'.
Calculated as: ½(109.5° - 60°) ≈ 24.75°.

4. Baeyer assumed that all cycloalkane rings are:

a) Puckered b) Planar c) Chair-shaped d) Linear

Answer: b) Planar.
A major limitation of the theory was the assumption that all rings are flat (planar).

5. Which theory explains the stability of higher cycloalkanes (like Cyclohexane) by introducing non-planar structures?

a) VSEPR Theory b) Baeyer Strain Theory c) Sachse-Mohr Theory d) Molecular Orbital Theory

Answer: c) Sachse-Mohr Theory.
They proposed that rings with 6 or more carbons pucker to relieve strain.

6. The internal bond angle of Cyclobutane is:

a) 60° b) 108° c) 120° d) 90°

Answer: d) 90°.
Cyclobutane is a square (planar assumption), so the internal angle is 90°.

7. Positive angle strain implies:

a) Bond angle is greater than 109° 28' b) Bond angle is less than 109° 28' c) No strain exists d) Molecule is linear

Answer: b) Bond angle is less than 109° 28'.
When the bond angle is compressed (smaller than ideal), the strain is positive (inward compression).

8. Which of the following has the highest heat of combustion per -CH₂- group (indicating highest strain)?

a) Cyclohexane b) Cyclopentane c) Cyclopropane d) Cyclobutane

Answer: c) Cyclopropane.
Due to maximum angle strain, Cyclopropane has the highest energy and heat of combustion per methylene group.

9. In reality, which conformation of cyclohexane is the most stable?

a) Boat b) Twist Boat c) Chair d) Half-Chair

Answer: c) Chair.
The chair conformation is almost completely free of angle and torsional strain.

10. Why did Baeyer's theory fail for Cyclohexane?

a) It ignored bond length b) It assumed the ring was planar (angle 120°) c) Carbon valency was wrong d) It didn't account for hydrogen atoms

Answer: b) It assumed the ring was planar (angle 120°).
A planar hexagon would have 120° angles, causing negative strain. Real cyclohexane puckers to achieve 109.5°.

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